Toothed elastomeric endless power transmission belts are well known and widely used in industry due to their high flexibility and sheave-engaging surfaces of maximum area which enable greater power transmission. However, each of such toothed belts is very noisy during operation because it produces a "siren-like" noise caused by the abrupt manner in which the teeth thereof enter and exit associated sheaves and by grooves containing air columns which also enter and exit the associated sheaves in an abrupt manner.